1. Field of the Invention
This invention relates generally to a cannula used as an aid for insertion of catheters and other instruments into the body and more particularly to a cannula that is easily removed after the insertion is completed.
2. Brief Description of the Prior Art
It is desirable that after a catheter or other body probe device has been inserted into place within a blood vessel that the apparatus used to aid the insertion is easily removable. When sharp devices such as hollow needles are left in place while the catheter or other device is being utilized, there is a danger that the needle may pierce through the vessel, causing tissue damage and rendering the catheter useless. In assemblies where the needle is removable, this problem is alleviated; however, in these cases, the cannula which is used as a passageway into the blood vessel, often cannot itself be easily removed when the probe or catheter that has been inserted has an enlarged proximal end. The inability to easily remove the cannula is a disadvantage in circumstances when the inserted probe or catheter must remain in the body for an extended period of time to be permanently implanted.
Various removable devices have been developed using frangible, hollow needles (see Rubin, U.S. Pat. No. 3,382,872; MacGregor, U.S. Pat. No. 3,550,591; Warren, U.S. Pat. No. 3,598,118 and Nerz, U.S. Pat. No. 3,677,243). In these devices the hollow needle must have a larger diameter to accommodate the catheter within the device. The larger size of the hollow needles can be a cause of additional tissue damage. Because of their rigidity and sharpness, these needles act as poor cannuli in situations where probes must be inserted and removed repeatedly, and they cannot be used at all when the cannula must be used as an aid in advancing the probe along the interior of the blood vessel.
Ansari, U.S. Pat. No. 3,545,443, utilizes dual hollow concentric needles, each having a longitudinal slot. After insertion, the needle assembly is removed and twisted in such a manner that the slots are aligned. The catheter then can pass through the slots thus removing the needle. This non-frangible assembly possesses many of the same disadvantages that the other hollow needle devices have. In addition, because there are two hollow needles, an even larger needle diameter is necessary to accomodate the same size catheter. The slots in the needles may also cause additional tissue damage and potentially may not be able to effectively prevent blood loss and air emboli.
Gauthier, U.S. Pat. No. 3,094,122, uses a percutaneous needle that is inside the catheter. After insertion, the needle is removed lengthwise from the catheter. There are several restrictions upon this method. The needle must be longer than the catheter and the method is only adaptable for insertion of hollow devices or catheters and cannot be used where various probes and leads are to be inserted. Guttman, U.S. Pat. No. 3,225,762, is similar to the Gauthier device except that the catheter has a slot for removal of the needle after insertion. This eliminates the need to have a needle that is longer than the catheter, however, there may be undesirable leakage through the slit, particularly when the catheter is bent at or near the slit. This invention also cannot be used where the probe to be inserted within the vessel is not hollow.
Reilly, U.S. Pat. No. 3,570,485, provides a flexible cannula, placed over the percutaneous needle, that is slotted along its length. After insertion, the needle is removed and the cannula can then act as a sheath for insertion of the catheter or other device. After insertion of the probe, the cannula can be removed by sliding the catheter or other device through the slit in the cannula. Although this invention is flexible and can be adopted for insertion of both catheters and non-hollow probes, the longitudinal slot may not provide a blood or airtight seal. This problem may be reduced by increasing the thickness of the cannula, however this would necessitate either the use of a larger puncture diameter or a smaller probe. Further, removal of the cannula requires gripping both the cannula and the inserted catheter and physically pulling them apart. Applying such a force upon an object that is partially inserted into the body may cause tissue damage. There is also the potential of accidentally pulling the catheter out from its inserted position.
The Reilly patent has a further limitation in its collapsibility. When the Reilly cannula is empty prior to insertion of the catheter or probe, it collapses upon itself at the puncture site with the purpose of preventing blood loss. However, this attribute creates a disadvantage in that a stiff catheter with a stiff tapered distal end is required in order to reopen the cannula for reinsertion; therefore, flimsy and flexible leads, such as pacemaker leads and blunted tipped catheters, such as balloon tipped catheters, are unadoptable for use with this device.